The 1888 LDS edition added the preposition for at the beginning of the parenthetical clause in this passage (giving “and for this was their secret plan and their combination”). One could interpret “for this” as meaning ‘for this intent’. No subsequent LDS edition has followed this reading since that edition was never used as a copytext.
There are no other examples in the text of the prepositional phrase “for this” at the head of a sentence. There are, to be sure, examples of “for this N”, where this acts as modifier for an explicit noun N; in theory each of the following phrases could work here in Helaman 2:8:
“for this cause” 27 times
“for this intent” 6 times
“for this end” 2 times
“for this purpose” 1 time
The critical text will ignore the intrusive for in Helaman 2:8; if it were correct, we would expect a following noun such as cause or intent.
Don Brugger (personal communication) points out that the intrusive for in the 1888 LDS edition may have been intended as a replacement for the conjunction and. In other words, the 1888 typesetter momentarily thought to replace and with for in this parenthetical statement (“for this was their secret plan and their combination”) but ended up adding the for without omitting the and.
Summary: Maintain in Helaman 2:8 the original reading, “and this was their secret plan and their combination”, without the preposition for that the 1888 LDS edition introduced before the this.